Basal or basilar crackles (not to be confused with the basilar artery of the brain) are crackles apparently originating in or near the base of the lung. Bibasal or bibasilar crackles refer to crackles at the bases of both the left and right lungs. Bilateral basal crackles also refers to the presence of basal crackles in both lungs.

Crackles are caused by the "popping open" of small airways and alveoli collapsed by fluid, exudate, or lack of aeration during expiration. The word "rales" derives from the French word râle meaning "rattle".

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René Laennec developed the term 'râles' ('rattles' in French) to describe the added breath sounds which are now referred to as 'crackles'. He described them using unusual daily examples, such as 'whistling of little birds', 'crackling of salt on a heated dish', 'cooing of the woodpidgeon' etc. but soon realised that he was unable to use this term in front of his patients because it conjured the association of 'le râle de la mort', which translates to 'death rattle' i.e. the noise that people who are about to die make when they can no longer clear secretions. Therefore at the bedside, he used the Latin equivalent, 'rhonchus'. This was not clearly understood by his translator, John Forbes, and the terminology became very confusing after the publication of De L'Auscultation Mediate. Therefore in 1977, a standardization was established by the American Thoracic Society and American College of Chest Physicians.[1] As a result of this, the term 'râles' was abandoned, and 'crackles' became its official substitute. Therefore these sounds should be correctly described as crackles.[1][2]

Crackles are caused by explosive opening of small airways[2] and are discontinuous,[3] nonmusical, and brief. Crackles are much more common during the inspiratory than the expiratory phase of breathing, but they may be heard during the expiratory phase. Crackles are often associated with inflammation or infection of the small bronchi, bronchioles, and alveoli. Crackles that do not clear after a cough may indicate pulmonary edema or fluid in the alveoli due to heart failure, pulmonary fibrosis, or acute respiratory distress syndrome. Crackles that partially clear or change after coughing may indicate bronchiectasis.

Crackles are often described as fine, medium, and coarse. They can also be characterized as to their timing: fine crackles are usually late-inspiratory, whereas coarse crackles are early inspiratory.

Fine crackles are soft, high-pitched, and very brief. This sound can be simulated by rolling a strand of hair between one's fingers near the ears, or by moistening one's thumb and index finger and separating them near the ears. Their presence usually indicates an interstitial process, such as pulmonary fibrosis or congestive heart failure. The sounds from interstitial pulmonary fibrosis have been described as sounding like opening a Velcro fastener.

Coarse crackles are somewhat louder, lower in pitch, and last longer than fine crackles. Their presence usually indicates an airway disease, such as bronchiectasis.

They can also be described as unilateral or bilateral, as well as dry or moist/wet.[4]

Crackles can be heard over the lower lobe of the lungs. Pulmonary edema makes it much more audible.